I weigh 100 kgs. My bic Electric Rock is 102 l, 8,5kg and I sink it to my ankles but it's still possible to pull the rig up on it. My carbon RRD X-fire is 103 l, 7-8kgs and I sink it to my knees making it impossible to pull the rig up on. What gives? Did one or both manufacturers estimate incorrectly. Is RRD using less floaty material (sounds unlikely)?

"Floatiness" of a material is basically its density. DENSITY = WEIGHT / VOLUME. Therefore the "floatiness" of a board is the weight in kg/volume in litres. The bic should be 83 grams per litre. The RRD should be 77 grams per litre at worst (ie LESS dense, or more floaty litre for litre).

Total "bouyant force" would be the volume (i.e. weight of water displaced if the board was fully submerged) minus the weight of the board. BOUYANT FORCE = WEIGHT OF WATER DISPLACED - WEIGHT OF BOARD. So the Bic would displace 102kg of fresh water-8.5kg board weight = 93.5kg bouyant force. The RRD would be 103 - 8 = 95kg. I think incorrect estimates of weight and volume or both are possible.

Back in the day one of the American windsurf magazines used to give two measures of bouyancy in test boards: saltwater and fresh water (as boards will float better in salt water). This is because salt water is more dense (heavier per litre) than fresh water. Fresh water should be 1000 grams per litre, salt water is about 1020 grams per litre, or about 2% more dense- hence the weight of water displaced by a board of a given volume (if fully submerged) is greater in salt water.

Clarence

Or your RRD got flooded, have you weighed it lately?

At least one of the manufacturer's estimates is wrong. I remember one surf magazine that used to measure the actual volume of boards, and they often were 5 liters higher or lower than what they were supposed to be.
For the XFire, it's quite possible that they did not take into account the reduced volume from the lowered mast track section.

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boardsurfr said..
At least one of the manufacturer's estimates is wrong. I remember one surf magazine that used to measure the actual volume of boards, and they often were 5 liters higher or lower than what they were supposed to be.
For the XFire, it's quite possible that they did not take into account the reduced volume from the lowered mast track section.

What he said^

quoted volume isn't always the actual displaced volume

Select to expand quote

boardsurfr said..
At least one of the manufacturer's estimates is wrong. I remember one surf magazine that used to measure the actual volume of boards, and they often were 5 liters higher or lower than what they were supposed to be.
For the XFire, it's quite possible that they did not take into account the reduced volume from the lowered mast track section.

Spot on
Unfortunately it is so easy to get it right, I don't know why they can't.......

I suspect it of marketing involved too, 98L waveboard sounds like its for normal folks and 104L sounds big etc. They could in reality be the other way around. I just got a 104 and bit nervous about getting it as my same brand 100L (same thing but old model) was perfect size so I thought 104 may be too big...... It feels same or smaller than the 100!

Same thing with sails i suppose.
They could add or take away a cheeky little m2 to add more power or maneuverability .
Ive laid two 7.8 supposedly same size sails one on top of the other and i estimated at least 0.3m2 difference and that was on same brand and type sail , three years apart.

Yeah, high chance the volume is wrong. The other aspect is an Electric-Rock is a way longer board, and mast track is probably a lot in front? Maybe it plays a role on how fast or not the board sink since you have to counter balance for the mast track?
Do you stay tall for a few second, than the board sink to his max..than uphaul...or when you have the sail up..you sink up to the ankle...probably volume is wrong.

Wasn't one of the manufacturers using "apparent volume" when boards first went wide.

I bought an older 76L board and the manufacturers blog site confirmed that by the time the prototype 76L went to production they were closer to 80L.